MySQL数据库网卡软中断不平衡问题及解决方案

　　最近公司在MySQL的数据库上由于采用了高速的如PCIe卡以及大内存，去年在压力测试的时候突然发现数据库的流量可以把一个千M网卡压满了。随着数据库的优化，现在流量可以达到150M，所以我们采用了双网卡，在交换机上绑定，做LB的方式，提高系统的吞吐量。

　　但是在最近压测试的一个数据库中，mpstat发现其中一个核的CPU被软中断耗尽：

　　Mysql QPS 2W左右

——– —–load-avg—- —cpu-usage— —swap— -QPS- -TPS- -Hit%-
time | 1m 5m 15m |usr sys idl iow| si so| ins upd del sel iud| lor hit|
13:43:46| 0.00 0.00 0.00| 67 27 3 3| 0 0| 0 0 0 0 0| 0 100.00|
13:43:47| 0.00 0.00 0.00| 30 10 60 0| 0 0| 0 0 0 19281 0| 326839 100.00|
13:43:48| 0.00 0.00 0.00| 28 10 63 0| 0 0| 0 0 0 19083 0| 323377 100.00|
13:43:49| 0.00 0.00 0.00| 28 10 63 0| 0 0| 0 0 0 19482 0| 330185 100.00|
13:43:50| 0.00 0.00 0.00| 26 9 65 0| 0 0| 0 0 0 19379 0| 328575 100.00|
13:43:51| 0.00 0.00 0.00| 27 9 64 0| 0 0| 0 0 0 19723 0| 334378 100.00|

　　mpstat -P ALL 1说：　　

　　针对这个问题，我们利用工具，特别是systemtap, 一步步来调查和解决问题。

　　首先我们来确认下网卡的设置：

$uname -r
2.6.32-131.21.1.tb399.el6.x86_64
$ lspci -vvvv
01:00.0 Ethernet controller: Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet (rev 20)
        Subsystem: Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet
        Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 0, Cache Line Size: 256 bytes
        Interrupt: pin A routed to IRQ 114
        Region 0: Memory at f6000000 (64-bit, non-prefetchable) [size=32M]
        Capabilities: <access denied>
01:00.1 Ethernet controller: Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet (rev 20)
        Subsystem: Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet
        Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B-
        Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR-
        Latency: 0, Cache Line Size: 256 bytes
        Interrupt: pin B routed to IRQ 122
        Region 0: Memory at f8000000 (64-bit, non-prefetchable) [size=32M]
        Capabilities: <access denied>
$cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.6.0 (September 26, 2009)
Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: em1
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 0
Down Delay (ms): 0
Slave Interface: em1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 78:2b:cb:1f:eb:c9
Slave queue ID: 0
Slave Interface: em2
MII Status: up
Link Failure Count: 0
Permanent HW addr: 78:2b:cb:1f:eb:ca
Slave queue ID: 0

　　从上面的信息我们可以确认二块 Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet (rev 20)网卡在做bonding。

　　我们的系统内核组维护的是RHEL 6.1, 很容易可以从/proc/interrupts和/proc/softirqs得到中断和软中断的信息的信息。

　　我们特别留意下softirq, 由于CPU太多，信息太乱，我只列出7个核心的情况：

$cat /proc/softirqs|tr -s ‘ ‘ ‘t’|cut -f 1-8
        CPU0    CPU1    CPU2    CPU3    CPU4    CPU5    CPU6
        HI:     0       0       0       0       0       0
        TIMER:  401626149       366513734       274660062       286091775       252287943       258932438
        NET_TX: 136905  10428   17269   25080   16613   17876
        NET_RX: 1898437808      2857018450      580117978       26443   11820   15545
        BLOCK:  716495491       805780859       113853932       132589667       106297189       104629321
BLOCK_IOPOLL:   0       0       0       0       0       0       0
        TASKLET:        190643874       775861235       0       0       1       0
        SCHED:  61726009        66994763        102590355       83277433        144588168       154635009
        HRTIMER:        1883420 1837160 2316722 2369920 1721755 1666867
        RCU:    391610041       365150626       275741153       287074106       253401636       260389306

　　从上面我们粗粗可以看出网卡的软中断接收和发送都不平衡。

　　单单这些信息还不够，还是无法区别为什么一个核心被压垮了，因为我们的机器上还有个中断的大户:fusionIO PCIe卡，在过去的测试中该卡也会吃掉大量的CPU，所以目前无法判断就是网卡引起的，因而我们用stap来double check下：

$cat i.stp
global hard, soft, wq
probe irq_handler.entry {
hard[irq, dev_name]++;
}
probe timer.s(1) {
println(“==irq number:dev_name”)
foreach( [irq, dev_name] in hard- limit 5) {
printf(“%d,%s->%dn”, irq, kernel_string(dev_name), hard[irq, dev_name]);
}
println(“==softirq cpu:h:vec:action”)
foreach( 1 in soft- limit 5) {
printf(“%d:%x:%x:%s->%dn”, c, h, vec, symdata(action), soft1);
}
println(“==workqueue wq_thread:work_func”)
foreach( [wq_thread,work_func] in wq- limit 5) {
printf(“%x:%x->%dn”, wq_thread, work_func, wq[wq_thread, work_func]);
}
println(“n”)
delete hard
delete soft
delete wq
}
probe softirq.entry {
soft[cpu(), h,vec,action]++;
}
probe workqueue.execute {
wq[wq_thread, work_func]++
}
probe begin {
println(“~”)
}
$sudo stap i.stp
==irq number:dev_name
73,em1-6->7150
50,iodrive-fct0->7015
71,em1-4->6985
74,em1-7->6680
69,em1-2->6557
==softirq cpu:h:vec:action
1:ffffffff81a23098:ffffffff81a23080:0xffffffff81411110->36627
1:ffffffff81a230b0:ffffffff81a23080:0xffffffff8106f950->2169
1:ffffffff81a230a0:ffffffff81a23080:0xffffffff81237100->1736
0:ffffffff81a230a0:ffffffff81a23080:0xffffffff81237100->1308
1:ffffffff81a23088:ffffffff81a23080:0xffffffff81079ee0->941
==workqueue wq_thread:work_func
ffff880c14268a80:ffffffffa026b390->51
ffff880c1422e0c0:ffffffffa026b390->30
ffff880c1425f580:ffffffffa026b390->25
ffff880c1422f540:ffffffffa026b390->24
ffff880c14268040:ffffffffa026b390->23
#上面软中断的action的符号信息：
$addr2line -e /usr/lib/debug/lib/modules/2.6.32-131.21.1.tb411.el6.x86_64/vmlinux ffffffff81411110
/home/ads/build22_6u0_x64/workspace/kernel-el6/origin/taobao-kernel-build/kernel-2.6.32-131.21.1.el6/linux-2.6.32-131.21.1.el6.x86_64/net/core/ethtool.c:653
$addr2line -e /usr/lib/debug/lib/modules/2.6.32-131.21.1.tb411.el6.x86_64/vmlinux ffffffff810dc3a0
/home/ads/build22_6u0_x64/workspace/kernel-el6/origin/taobao-kernel-build/kernel-2.6.32-131.21.1.el6/linux-2.6.32-131.21.1.el6.x86_64/kernel/relay.c:466
$addr2line -e /usr/lib/debug/lib/modules/2.6.32-131.21.1.tb411.el6.x86_64/vmlinux ffffffff81079ee0
/home/ads/build22_6u0_x64/workspace/kernel-el6/origin/taobao-kernel-build/kernel-2.6.32-131.21.1.el6/linux-2.6.32-131.21.1.el6.x86_64/include/trace/events/timer.h:118
$addr2line -e /usr/lib/debug/lib/modules/2.6.32-131.21.1.tb411.el6.x86_64/vmlinux ffffffff8105d120
/home/ads/build22_6u0_x64/workspace/kernel-el6/origin/taobao-kernel-build/kernel-2.6.32-131.21.1.el6/linux-2.6.32-131.21.1.el6.x86_64/kernel/sched.c:2460

　　这次我们可以轻松的定位到硬中断基本上是平衡的，软中断都基本压在了1号核心上，再根据符号查找确认是网卡的问题。

　　好了，现在定位到了，问题解决起来就容易了：

　　1. 采用多队列万M网卡。

　　2. 用google的RPS patch来解决软中断平衡的问题, 把软中断分散到不同的核心去，参见这里.

　　我们还是用穷人的方案，写了个shell脚本来做这个事情：

$cat em.sh
#! /bin/bash
for i in `seq 0 7`
do
  echo f|sudo tee /sys/class/net/em1/queues/rx-$i/rps_cpus >/dev/null
  echo f|sudo tee /sys/class/net/em2/queues/rx-$i/rps_cpus >/dev/null
done
$sudo ./em.sh
$mpstat -P ALL 1

　　就可以看到我们的成果：